Polyvinyl butyral (PVB, CAS nr 63148-65-2) is a polymer prepared from polyvinyl alcohol by reaction with butyric aldehyde. Plasticized PVB has a glass transition temperature of approximately 10-28° C., depending on the used plasticizer, and other factors. Unplasticized PVB has a glass transition temperature of approximately 62° C. At temperatures well below the glass transition temperature, the PVB becomes glass-like and brittle. Above the glass transition temperature, the PVB material becomes flexible and rubbery
Polyvinyl butyral is mostly used for applications where strong adhesion, transparency, toughness and flexibility are required. The major application is laminated glass, in particular, laminated safety glass for car windshields or for use in creating furniture or used in buildings and/or architectural creations. Other emerging applications are protective films e.g. for solar panels.
Common trade names for PVB-film materials include Everlam, KBPVB, Saflex, GlasNovations, Butacite, Winlite, S-Lec, Trosifol and many others.
PVB is a valuable polymer and can be recycled and reused in new applications if the remaining glass particulates as well as components of the PVB fraction, such as plasticizers, degradation products and/or other additives are separated or removed from the recovered PVB fraction prior to reusing in another application. An example of such novel application is by using recovered PVB e.g. as carpet backing material.
Recycling requires effective separation from windshield glass, removal of metal, dust, sand, glass and other particulate materials including also removing colored PVB particles or flakes. Further, certain additives and plasticizers (see definitions above) including VOC's (volatile organic compounds) must be removed as they give rise to negative mechanical properties and negative olfactory sensations, i.e. unpleasant smell, either directly or following sunlight exposure, hydrolysis and/or microbial actions.
In the present application, CO2 or carbon dioxide is to be understood as supercritical CO2, i.e. CO2 above 35° C. and above 70 bars, or liquid CO2, i.e. typically between −20 to +30° C. and at 30-65 bars.
Plasticizer for PVB is in principle any additive which softens PVB and imparts favorable properties for PVB in commercial operations such as intermediate layer in laminated glass, as base polymer in protective films, carpet backing applications or other polymer applications. A wellknown plasticizer for PVB is for example TEG-EH (triethylene glycol ester of 2-ethylhexanoic acid).
In the present application the terms degradation products or contaminants encompass the group of compounds that appears when PVB is degraded as already indicated above or especially under humid and weak acidic conditions e.g. because of being stored on a waste tip for windscreens and/or laminate. Similarly, the PVB may be stored on a waste tip after the laminated glass products have been subjected to preliminary separation step resulting in a glass fraction and a PVB fraction containing the PVB interlayer including some glass particles.
The group of compounds that appears upon degradation comprise mostly butyric acid, butyric aldehyde, butoxyethanol, polyvinyl alcohol monomers and oligomers, but may also encompass other degradation products that are presently not identified.
Treatment of polymers in high pressure fluids such as liquid CO2 or supercritical CO2 is well known. Specifically, for PVB, U.S. Pat. No. 5,739,270 B (Farmer et al, assigned to CF Technologies) teaches the extraction of plasticizer and separation of undesired particles and materials from liquid PVB fractions using “critical” (liquid) or supercritical CO2. Further, in order to prepare a liquid PVB composition, an organic solvent (described as a “cosolvent”), such as methanol, acetone or similar is added to the PVB fraction in which “which at least partially dissolves the polymer”. The purpose of adding the cosolvent is to reduce the viscosity of the polymer slurry in order to allow subsequent filtration. The use of organic solvents in the PVB recovery process results in a health risks in relation to the workers who are working in the facility where the PVB recovery is performed. This health risk and general risk of leakage is increased since the CO2 extraction is carried out at relatively high pressure because of the use of liquid or supercritical CO2. In addition, the use of organic solvents may also cause an increased risk of fires in case of leakage of organic solvents.
Thus, the currently available recovery and upgrade methods provide a product that is ready for reuse, but the recovery methods suffer from serious drawbacks:                i) organic solvents are required, and their use is increasingly being viewed as unacceptable and too costly,        ii) ii) the CO2 extraction is lengthy remove plasticisers. Besides the cosolvents need to be properly separated from the upgraded PVB in order to ensure that the resulting upgraded PVB fraction does not contain organic solvents that evaporate and cause fire/and/or health risks,        iii) in practice the level of extraction of plasticisers cannot be controlled, with the risk of being forced to add necessary plasticizer after the process. This may include using expensive twin-screw extruders.        
In general, the proposed technology in U.S. Pat. No. 5,739,270 B is too costly and too time consuming in view of normal economic considerations.
Thus, there is a need for alternative solutions for upgrading PVB post-industrial and/or post-consumer waste fractions of PVB, e.g. from interlayers of laminated glass or other applications to provide a recovered product which is applicable for reuse in other industries or for reuse in preparing new laminated glass products.